Field of the Invention
This invention relates to flow field plate modifications for fuel cells in order to improve coolant distribution. It particularly relates to modifications for bipolar plate assemblies for solid polymer electrolyte fuel cells.
Description of the Related Art
Fuel cells such as solid polymer electrolyte or proton exchange membrane fuel cells electrochemically convert reactants, namely fuel (such as hydrogen) and oxidant (such as oxygen or air), to generate electric power. Solid polymer electrolyte fuel cells generally employ a proton conducting, solid polymer membrane electrolyte between cathode and anode electrodes. A structure comprising a solid polymer membrane electrolyte sandwiched between these two electrodes is known as a membrane electrode assembly (MEA). In a typical fuel cell, flow field plates comprising numerous fluid distribution channels for the reactants are provided on either side of a MEA to distribute fuel and oxidant to the respective electrodes and to remove by-products of the electrochemical reactions taking place within the fuel cell. Water is the primary by-product in a cell operating on hydrogen and air reactants. Because the output voltage of a single cell is of order of 1V, a plurality of cells is usually stacked together in series for commercial applications in order to provide a higher output voltage. Fuel cell stacks can be further connected in arrays of interconnected stacks in series and/or parallel for use in automotive applications and the like.
Along with water, heat is a significant by-product from the electrochemical reactions taking place within the fuel cell. Means for cooling a fuel cell stack is thus generally required. Stacks designed to achieve high power density (e.g. automotive stacks) typically circulate liquid coolant throughout the stack in order to remove heat quickly and efficiently. To accomplish this, coolant flow fields comprising numerous coolant channels are also typically incorporated in the flow field plates of the cells in the stacks. The coolant flow fields may be formed on the electrochemically inactive surfaces of the flow field plates and thus can distribute coolant relatively evenly throughout the cells while keeping the coolant reliably separated from the reactants.
Bipolar plate assemblies comprising an anode flow field plate and a cathode flow field plate which have been appropriately sealed and bonded together so as to form a sealed coolant flow field between the plates are thus commonly employed in the art. The plates making up such assemblies may optionally be metallic with appropriate corrosion resistant coatings and are typically produced by stamping the desired features into sheets of appropriate metal materials (e.g. certain stainless steels). Alternatively, the plates may be carbonaceous and are typically produced by molding features into plates made of appropriate moldable carbonaceous materials (e.g. polymer impregnated expanded graphite).
To provide both reactants and the coolant to and from the individual cells in the stack, a series of ports are generally provided at opposing ends of the plates for individual cells such that when the cells are stacked together they form manifolds for these fluids. Further design features that may be required then are passageways to distribute the bulk fluids to and from the various channels in the reactant and coolant flow field channels in the plates. These passageway regions are referred to as the transition regions. For instance, such regions associated with the coolant are referred to as the coolant transition regions. The various transition regions can themselves comprise numerous fluid distribution channels, e.g. fuel transition channels in a fuel transition region.
For ease of manufacture and other reasons, a common stack design employs a stack of generally rectangular, planar fuel cells whose flow field plates comprise numerous straight reactant and coolant flow field channels running from one end of the plates to the other. Further, it can be advantageous to employ a stack configuration in which certain of the ports are located on the side of the plates and thus are not in line with the flow field channels. Such a configuration however necessitates directing the associated fluid transverse to the flow field channels in order to fluidly connect the ports to the transition regions and then to the flow field channels. In designs in which transition channels appear in the transition regions, this can be accomplished by forming a duct or ducts transverse to the transition channels. As will be more apparent when discussing the Figures below, the presence of such a duct or ducts can impede the flow of other fluids in a coolant transition region. Thus, a trade-off can be required between flow through such ducts and flow through other transition channels.
Such a trade-off can be of particular concern in high power density stack designs that comprise coolant ports located on the side or sides of the plates and coolant ducts in the coolant transition region that are directed transverse to the flow fields. To achieve the highest power densities, fluid channels are often formed at the limits of reliable manufacturing capability and tolerances. Being a liquid, the coolant flow is subject to greater pressure drops than a gaseous reactant when flowing through ducts or channels of a given size. As a result, the coolant pressure drop can be particularly significant in the coolant transition regions of such high power density stacks, and especially in wider cells where longer transverse coolant ducts must be employed. Further, temperature gradients may be created across the coolant duct as the coolant traverses the coolant duct. As a consequence, the pressure and temperature of the coolant in the coolant duct may not be as uniform as desired, and thus the flow and temperature of coolant in the coolant flow field channels as it traverses the fuel cell may not be as uniform as desired. Any non-uniformity however can adversely affect cell performance in various ways. For instance, there can be an increased risk of overheating (hot spots) and over-drying in the cells on hot days. Also, it can lead to formation of wet spots in the cells, making it difficult to prepare the stack for shutdown in below freezing conditions and also difficult to recover during startup from below freezing conditions.
Despite the advances made to date, there remains a need for ever greater power density from fuel cell stacks while maintaining performance over a wide range of operating conditions. Providing greater uniformity over the active regions in the fuel cell stacks makes it possible to coax closer to the optimum performance out of the entire available area in these active regions. This invention fulfills these needs and provides further related advantages.